This invention relates to the field of microfabricated devices and, in particular, to the thermal isolation of such structures from a substrate utilizing vertically built supports.
Certain types of microfabricated devices require thermal isolation from the substrate upon which they are fabricated. One good example of this is a sensor used to detect infrared energy, which is implemented as a thermal bi-morph. Thermal bi-morph devices are typically cantilever beams that are made of two components having different thermal expansion coefficients. When the beam is heated, the difference in thermal expansion coefficients causes the beam to bend. The amount of bending can then be related to the received infrared energy. The concept of the use of a thermal bi-morph for infrared detection is not new.
Absent thermal isolation from the substrate, the heat in the bi-morph would be conducted into the substrate before a meaningful measurement of the bending of the bi-morph could be acquired.
Prior art examples of uncooled infrared sensors utilizing thermal bi-morphs are known. Such prior art examples, however, use a horizontal thermal isolation structure, as shown in FIG. 1. Typically, such horizontal thermal isolation structures are composed of a material having a low thermal conductivity, such as amorphous silicon carbide or silicon nitride.
One problem with such horizontal thermal isolation structures is the amount of wafer real estate required. The horizontal thermal isolation structure limits the number of sensors per unit of area on the wafer because of the additional wafer real estate required for the isolation structure. Because it is often desired to pack as many sensors on a wafer as possible, it is a goal to eliminate the horizontal thermal isolation structure in favor of a more compact design, preferably one that does not add to the wafer real estate required for any given sensor.
A novel approach to the fabrication of uncooled infrared sensors utilizing a vertical thermal isolation structure is described herein. As stated previously, thermal isolation structures provide necessary substrate/sensor thermal isolation to improve the sensitivity of the sensor to infrared energy. The benefit of vertical isolation structures is that they provide necessary thermal isolation while consuming a minimum amount of wafer real estate. Minimizing wafer real estate per sensor means that more sensors can be packed into a given unit area. If such infrared sensors are used, for example, for imaging, this means that the number of pixels available for imaging can be increased.
In the preferred embodiment, the thermal isolation structure is fabricated as a vertical structure contacting the substrate at one end and the infrared sensor, or any other microfabricated device requiring thermal isolation from the substrate, on the other end. Preferably, the microfabricated device, such as the bi-morph used in an infrared sensor, is cantilevered from the vertical thermal isolation structure. As a result, the vertical thermal isolation structure is located under the microfabricated device and does not take up any more, or very little more, wafer real estate that the actual device which is being thermally isolated from the substrate.
Several embodiments of vertical thermal isolation structures are disclosed herein. These include an L-shaped structure, a corrugated L-shaped structure, and a hollow tube structure. Various shaped structures may have advantages over other shapes, dependent upon the application. The disclosed shapes are meant to be illustrative only. Certainly other shapes may be utilized and fabricated with the methods described herein, and would therefore be within the scope and spirit of this invention. Also, this invention is not meant to be limited to bi-morphs for sensing infrared energy. The vertical thermal isolation structures and related fabrication methods can be used with any microfabricated devices requiring thermal isolation from the substrates on which they are built.